What Can Be the Cause of Hair Loss? What the Research Says

Hair loss has many possible causes, and the right explanation depends on the pattern, the timing, and who is affected. The most common cause is hereditary androgenetic alopecia, which affects roughly half of men by middle age and a large share of women after menopause. Other frequent causes include telogen effluvium (shedding after stress, illness, or childbirth), thyroid disease, iron deficiency, scalp infections, autoimmune alopecia areata, and reactions to certain medications or treatments.

Hair grows in cycles. Each follicle moves through a growth phase (anagen), a transition phase (catagen), and a resting phase (telogen), then sheds and begins again. Most scalp hairs are in the growth phase at any one time. When something disrupts that cycle — hormones, illness, nutrient shortages, inflammation — the result can be excess shedding, thinning, or patches of bare skin. Sorting out which mechanism is at work is what determines whether a cause is reversible.

What Is the Most Common Cause of Hair Loss?

Androgenetic alopecia — often called male-pattern or female-pattern hair loss — is the single most common cause. It is driven by genetics and by the effect of dihydrotestosterone (DHT), a hormone converted from testosterone, on hair follicles that are genetically sensitive to it.

In susceptible follicles, DHT causes the growth phase to shorten and the hair to become progressively finer and shorter. Over time the follicle itself shrinks. This is why pattern hair loss tends to be gradual and follows a recognizable shape — receding at the temples and crown in men, and a widening part or diffuse thinning over the top of the scalp in women.

Hereditary hair loss is not simply a matter of one gene from your mother’s father. That idea is a persistent myth. Research has identified many genetic variants that influence risk, and they come from both sides of the family. Having relatives with hair loss raises your likelihood, but it does not guarantee it.

Can Stress Be the Cause of Hair Loss?

Yes, but the type of stress matters and the timing surprises most people. The shedding caused by a stressful event usually appears weeks to a few months after the event, not during it.

This is called telogen effluvium. A physical or emotional stressor — surgery, a severe illness, a high fever, major weight loss, or significant psychological stress — can push a large number of follicles into the resting phase at once. When those hairs reach the end of that phase, they shed together. People often notice handfuls of hair on the pillow or in the shower.

Telogen effluvium is usually temporary. Once the trigger resolves, the cycle tends to normalize and hair regrows, though it can take several months. The key point is that the shedding is diffuse — spread across the scalp — rather than confined to patches or a receding hairline.

What Medical Conditions Can Cause Hair Loss?

Several medical conditions cause hair loss, and many are treatable once identified. This is why sudden or unexplained shedding deserves a medical evaluation rather than a cosmetic fix.

• Thyroid disorders. Both an overactive and an underactive thyroid can cause diffuse thinning. Hair typically recovers once thyroid levels are corrected.
• Iron deficiency. Low iron stores are a well-recognized contributor to hair shedding, particularly in women. Iron status is assessed with blood tests, not guessed at.
• Alopecia areata. An autoimmune condition in which the immune system attacks hair follicles, producing smooth, round patches of hair loss. It can affect the scalp, beard, eyebrows, or other body hair.
• Scalp infections. Fungal infections such as tinea capitis can cause patches of hair loss with scaling or broken hairs, and they require treatment.
• Nutritional deficiencies. Beyond iron, inadequate protein intake and deficiencies in certain vitamins and minerals can affect hair. Deficiencies should be confirmed by testing, not assumed.
• Hormonal conditions. Polycystic ovary syndrome and other hormonal imbalances are associated with hair thinning in women.

One clarification worth making: taking supplements for a deficiency you do not have is unlikely to help and is not without risk. High doses of some vitamins and minerals can cause harm. Testing comes first.

Can Medications and Treatments Cause Hair Loss?

Yes. Many medications list hair loss as a possible side effect, and this is a well-documented cause. The list is long, but some categories are more commonly implicated than others.

Certain chemotherapy drugs are well known for causing hair loss, often dramatically. Other medications — including some used for blood pressure, depression, acne, and blood thinning — can cause shedding in some people. Hormonal treatments and anabolic steroids can also affect hair.

If you suspect a medication is causing shedding, the right step is to talk to your prescriber. Do not stop a prescribed medication on your own, especially one for a heart condition, blood pressure, or mental health. The timing of shedding relative to starting a drug is a useful clue for your clinician.

Does Hair Loss Differ Between Men and Women?

Yes, both in pattern and in the range of likely causes. Men most often present with hereditary pattern loss — a receding hairline and thinning crown. Women more often notice diffuse thinning, a widening part, or increased shedding, and hormonal and nutritional causes are more frequently part of the picture.

Women can also experience a distinct type of shedding after childbirth, called postpartum telogen effluvium. It is common and typically temporary, driven by the sharp drop in hormones after delivery. Hair usually recovers over several months, though the timing varies from person to person.

Menopause is another point at which many women notice changes in hair thickness. The hormonal shifts of this stage are associated with thinning in some women, but the evidence on exactly how and how much is less clear than for hereditary pattern loss.

When Should Hair Loss Prompt a Medical Visit?

Sudden hair loss, patchy loss, loss accompanied by scalp symptoms, or shedding that persists should prompt a medical visit. The same is true for hair loss in a child, or any loss that is accompanied by fatigue, weight change, or other unexplained symptoms.

Rapid or patchy loss is different from gradual thinning. Sudden shedding, smooth bald patches, scaling, redness, or itching all point toward a cause that a clinician can evaluate. Some of these conditions — such as alopecia areata and scalp infections — have specific treatments, and early attention tends to be better than waiting.

There is no single test for hair loss. A clinician typically starts with history and a scalp exam, and may order blood tests for thyroid function, iron status, or other markers depending on the pattern. In some cases a small scalp biopsy or a hair-pull test helps clarify the diagnosis.

What About Diet, Smoking, and Hair Loss?

Severe nutritional deficiency can cause hair loss, and correcting a confirmed deficiency can help. Beyond that, the evidence linking specific diets or supplements to improved hair growth in people without deficiencies is limited. No clinical evidence currently confirms that a particular supplement regrows hair in someone with normal nutrient levels.

Smoking is associated with hair loss in some observational studies, though the evidence is not as strong as for genetics or hormones. The relationship is plausible given smoking’s effects on circulation, but it should not be presented as a proven cause. If you smoke, the broader health reasons to stop are far better established than the hair-specific ones.

Crash diets and rapid weight loss are a recognized trigger for telogen effluvium. That is one of the more practical connections between diet and shedding — it is the speed and severity of the change, not a single food, that tends to matter.